The Arabic satellite channel al-Jazeera said it would not show the video in full because of "humanitarian reasons".

But witnesses who have seen the tape said Mrs Hassan, 59, is shown fainting on camera after making an emotional plea for her release. She is then doused in water in an apparent attempt to revive her.

Al-Jazeera said the unknown terrorist group holding Mrs Hassan had threatened to hand her over to the al-Qa'eda-linked terrorist Abu Musab al-Zarqawi within 48 hours unless Britain withdraws troops from Iraq.

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Al-Zarqawi's Tawid and Jihad group has been responsible for most of the hostage murders in Iraq, including the beheading of Kenneth Bigley, the Briton killed last month.

Care International, for whom Mrs Hassan ran their Baghdad office, renewed its call for her release after the video was released.

"Mrs Hassan's colleagues are profoundly distressed by her condition and urge those holding her to release her without further harm," it said in a statement.

It added that Mrs Hassan, who is married to an Iraqi and also holds British and Irish nationality, is an Iraqi citizen who had dedicated her life to helping disadvantaged Iraqis.

The tape's contents were confirmed by Bertie Ahern, the Irish prime minister, who said he had not seen it but had read a transcript about the footage.

"A number of very dangerous and very serious things and time scales have been stated in that [video]," he said.

Mr Ahern later met Mrs Hassan's sisters, who have also seen the transcript.

Mrs Hassan was kidnapped by gunmen as she arrived for work in Baghdad last month. Previous videos of her have shown her in a distressed state appealing for her release and her captors' demands to be met.